skill_view accepted arbitrary file_path values like '../../.env' and
would read files outside the skill directory, exposing API keys and
other sensitive data.
Added two layers of defense:
1. Reject paths with '..' components (fast, catches obvious traversal)
2. resolve() containment check with trailing '/' to prevent prefix
collisions (catches symlinks and edge cases)
Fix approach from PR #242 (@Bartok9). Vulnerability reported by
@Farukest (#220, PR #221). Tests rewritten to properly mock SKILLS_DIR.
Closes#220
